Process for preparing silicon and optionally aluminum and silumin (aluminum-silicon alloy)

ABSTRACT

Process for preparing highly purified silicon and optionally aluminum and silumin (aluminum silicon alloy) in the same cell, wherein silicate and/or quartz containing rocks are subjected to electrolysis in a salt melt containing fluoride, whereby silicon and aluminum are formed in the same bath, and aluminum formed, which may be low alloyed, flow to the bottom and is optionally drawn off, and deposit formed on the cathode is removed from the cathode and crushed, optionally together with the remaining electrolysis bath, concentrated sulfuric acid and then hydrochloric acid and water are added to the crushed material, liberated Si-grains float to the surface and are taken out and treated further as desired.

This application is a 371 of PCT/NO02/00075 filed Feb. 21, 2002.

The present invention relates to a process for preparing silicon andoptionally aluminum and silumin (aluminum silicon alloy) in a salt meltby electrolysis and subsequent refining of the silicon. Silica andsilicate rocks and/or aluminum containing silicate rocks are used as rawmaterial, with/without soda (Na₂CO₃) and/or limestone (CaCO₃) dissolvedin fluorides, in particular cryolite.

The products prepared are of high purity.

WO 95/33870 (EP patent 763151), in the following designated as “WO 95”,discloses a process for continuous preparation and batch preparation inone or more steps in one or more furnaces, of silicon (Si), optionallysilumin (AlSi-alloys) and/or aluminum metal (Al) in a melting bath usingfeldspar or feldspar containing rocks dissolved in fluoride. In saidprocess Si of high purity is prepared by electrolysis (step I) in afirst furnace with a replaceable carbon anode arranged underneath thecathode, and a carbon cathode arranged at the top of the furnace. Forthe preparation of silumin the silicon-reduced residual electrolyte fromstep I is transferred to another furnace, and Al is added (step II).Then Al is prepared in a third furnace (step III) by electrolysis afterSi has been removed in step I and possibly in step II. It also describescombinations of furnaces with a partition wall in the preparation of thesame substances. Further, process equipment for the procedure isdescribed.

The present invention represents a further development and improvementof the above-mentioned process. The greatest improvement is that it ispossible to prepare pure Si, pure low-iron low-alloyed Al-alloys(AlSi-alloys) and pure low-phosphorus high-alloyed Al-alloys(SiAl-alloys) in the same furnace (step I) by varying such parameters asthe choice of raw material, current density (voltage) and time. Theproportions of the Si and Al-products are adjusted by the choice of rawmaterial and cathodic current density (voltage) in the electrolysis bathand mechanical manipulation of the cathodes. Further, the composition ofthe Al-products varies with the electrolysis time (examples 1–5).

A low-alloyed Al-alloy (AlSi-alloy) as referred to herein, is anAl-alloy with an amount of Si which is lower than that of an eutecticmixture (12% Si, 88% Al). Correspondingly, a high-alloyed Al-alloy(SiAl-alloy) as referred to herein is an alloy having a Si-content abovethat of an eutectic mixture.

According to the present invention there is provided a process forpreparing highly purified silicon and optionally aluminum and silumin(aluminum silicon alloy) in the same cell. The process takes place by

-   I. subjecting silicate and/or quartz containing rocks to    electrolysis in a fluoride containing salt melt, whereby silicon and    aluminum are formed in the same bath, and aluminum formed, which may    be low alloyed, flows downwards to the bottom and is optionally    drawn off, and-   II. deposit formed on the cathode of the electrolysis furnace is    removed from the cathode and crushed, optionally together with the    remaining electrolysis bath, and is treated with concentrated    sulfuric acid and then hydrochloric acid and water, and liberated    Si-grains float to the surface and are taken out and treated further    as desired.

Soda is added to the electrolysis bath so that said bath will be basicif quartz is used, in order to avoid loss of Si in the form of volatileSiF₄. With high concentrations of soda the melting point of the mixtureis reduced, and the use of added fluorides goes down. Limestone is addedif necessary to reduce the absorption of phophorus in the Si depositedon the cathode.

In connection with the further treatment (refining) of the Si-product,the fluorides in the salt melt should preferably be acidic. The acidicfluorides, which are formed by adding sulfuric acid to cryolite (stepII), have been analyzed and contain a mixture of cryolite (Na₃AlF₆) andaluminum fluoride (AlF₃). Possibly the mixture may be added externallyand stirred into molten silicon.

EXAMPLE 1 (from WO 95)

A feldspar of the type CaAl₂Si₂O₈ containing 50% SiO₂, 31% Al₂O₃ and0.8% Fe₂O₃, was dissolved in cryolite and electrolyzed with a cathodiccurrent density of 0.05 A/cm² (U=2.5–3.0 V) for 18.5 hours. In thedeposit around the cathode highly purified Si was formed separate fromsmall FeSi-grains. In the electrolyte dissolved Al₂O₃ was formed. Al isnot formed.

Since Al was not formed in the bath (Al³⁺-containing electrolyte) thiswas the reason why bath was drawn off from this furnace (step I) and toanother furnace (step II) in which residues of Si and Si(IV) wereremoved by addition of Al before the electrolysis and the preparation ofAl in a third furnace (step III). (See WO 95).

Conclusion: The reason why only Si and not Al was formed in step I inthe present case, was the low current density (voltage).

EXAMPLE 2

A feldspar of the type NaAlSi₃O₈, containing 68% SiO₂, 20% Al₂O₃ and0.07% Fe₂O₃, was dissolved in cryolite and electrolyzed with a cathodiccurrent density of 0.5 A/cm² (U=6.5–8.0 V) for 3 hours. In the depositaround the cathode highly purified Si and a few small FeSi-grains wereformed. Underneath the electrolyte Al (low-alloyed AlSi-alloy) wasformed, and this had low iron content.

Conclusion: The reason why both Si and Al were formed in step I was thehigh current density (voltage).

EXAMPLE 3

A diorite (rock) containing feldspar and quartz, analyzed to contain 72%SiO₂, 16% Al₂O₃ and 1.4% Fe₂O₃, was dissolved in cryolite andelectrolyzed at a cathodic current density of 0.5–1.6 A/cm² (U=2.5–8.0V) for 16.5 hours. In the deposit around the cathode highly purified Siand many small separate FeSi-grains were formed. Underneath theelectrolyte Al (low-alloyed AlSi-alloy) was formed, and this had a lowiron content.

Conclusion: The reason why both Si and Al were formed in step I was thehigh current density (voltage). The reason why the Al (AlSi-alloy) haslow iron content, is that the FeSi-grains remain in the deposit on thecathode.

EXAMPLE 4

A feldspar containing rock of the type KAlSi₃O₈, containing 65% SiO₂,18% Al₂O₃ and 0.3% Fe₂O₃, was dissolved in cryolite and electrolyzed ata cathodic current density of 0.5 A/cm² (U=3–4.0 V) for 13 hours. In thedeposit around the cathode highly purified Si and small FeSi-grains wereformed. Some of the deposit was pushed down into the bath (theelectrolyte). While the cathode deposit contained 20% Si, the bath (theelectrolyte) contained 3% Si after the final electrolysis. Underneaththe electrolyte Al (low-alloyed AlSi-alloy) was formed, and this stillhad a low content of iron.

Conclusion: The reason why both Si and Al were formed in step I is thehigh current density (voltage). The reason why the Al (the AlSi-alloy)still had a low content of iron, was that the FeSi-grains had not hadsufficient time to seep out of the viscous cathode deposit and into Albefore the bath was frozen.

EXAMPLE 5

Quartz containing close to 99.9% SiO₂ was dissolved in cryolite(Na₃AlF₆), mixed with 5% soda (Na₂CO₃) and electrolyzed with a cathodiccurrent density of 0.5 A/cm² (U=6–7 V) for 44 hours. In the depositaround the cathode highly purified Si was formed. Most of (12 kg) of thecathode deposit was pushed into the bath (the electrolyte). Theremaining cathode deposit (8 kg) was lifted out with the cathodestogether with the residues of the anode. The cathode deposit was easilyknocked off the cathodes and was mixed with the electrolyte in the bath.Both contained 20% Si. Small amounts of Al (low alloyed AlSi-alloy) wereformed, which were low in iron and phosphorus. Iron and phosphorus poorAlSi-alloys are defined as <130 ppm Fe and <8 ppm P. The analysis of Alshowed 8% Si and 110 ppm Fe and 0.08 ppm P.

Conclusion: The reason why both Si and Al were formed in step I was thehigh current density (voltage). Al originates from electrolyzedcryolite. The reason why Al (the AlSi-alloy) was now alloyed with Si,was that Si from the cathode deposit starts to dissolved in Al. Thereason why the Al-alloy is iron and phophorus poor is that the rawmaterials initially are low in iron and phophorus.

The above examples 1–5 illustrate step I of the present process.

The silicon together with residues of small grains of FeSi prepared byacid refining (step II), contains a total of 75 ppm Fe and about 15 ppmP. The concentrated Si powder mixture contained 80% Si or more. In afurther treatment in the form of crystal rectification of the siliconafter step II a distribution coefficient (segregation coefficient) of0.35 for phosphorus is expected. This means that when the Si powdercontained 15 ppm P it is expected that crystal rectified Si shouldcontain about 6 ppm P. In addition it was found that the crystallizationof Si was not perfect. From this one could conclude that the P-contentshould have been higher than 6 ppm. The analysis showed that theP-content in Si was 1.0 ppm. The reason why the P-content is so low isfound to be the mixing of slag with the fluorides, which takes placewith good stirring of the Si melt with slag. The silicon contained 3 ppmcontaminations or 99.9997% Si.

If it is desired to prepare Al together with Si, the cathodic currentdensity should be relatively high, at least above 0.05 A/cm², preferablyabove 0.1, in particular above 0.2 A/cm². An upper limit is about 2,preferably about 1.6 A/cm². In addition to the formation of aluminumwith a high current density, the electrolysis rate also increases withincreasing cathodic current density.

In all the described examples it was found that the purity of Si was inthe range 99.92–99.99%. Previously (WO 95) in order to concentrate Sifurther above 20% from the cathode deposit, the cathode deposit wascrushed so that as much as possible of free and partly not freeSi-grains would float up and could be taken up on the surface in a heavyliquid consisting of different C₂H₂Br₄/acetone mixtures with a densityof up to 2.96 g/cm³. Si in solid form has a density of 2.3 g/cm³ andwill float up, while solids of cryolite have a density of 3 g/cm³ andwill remain at a bottom. After filtration and drying of the powder forremoval of heavy liquid, the different concentration fractions weremixed with water/H₂SO₄/HCl for refining Si.

In WO 97/27143, in the following designated as “WO 97”, water, HCl andH₂SO₄ in this order were added to crushed cathode deposit, containing20% Si, to refine Si with a dilute NaOH which was formed by addingwater. Then it was tried to concentrate the powder containing Si refinedwith HCl, with concentrated H₂SO₄.

Neither in WO 95 nor in WO 97 was Si concentrated more than to about40%. The reason for this is that the fluorooxosilicate complexes in thecathode deposit were hydrolyzed in water and NaOH to form a difficultlysoluble hydrated silica. As a consequence of this an addition of H₂SO₄after the treatment with water did not accomplish the concentrationeffect which it has when added directly to untreated dry powder.Concentrated HCl does not have any essential concentrating effect as itcontains much water in contrast to concentrated H₂SO₄. In WO 97 a jigwas used to concentrate Si further. This resulted only in aninsignificant concentration.

When it is primarily desired to prepare Si, a quartz containing rock issuitably used as starting material. If Al is also of interest, a rockcontaining an Al-rich feldspar, for instance anorthite (CaAl₂Si₂O₈) issuitably used.

A new and essential feature of the invention is that concentrated H₂SO₄is added to the untreated, pulverized cathode deposit containing 20% Si,or the pulverized bath (electrolyte) containing 20% Si, or mixtures ofthese. The powder fractions initially result in a concentration of Si toabout 50% as the sulfuric acid has a good dissolving effect on cryolite.This mixture of 50% Si and other residual products, i.a. acidicsulfates, represents a sticky substance which must be treated further.By diluting the mixture with water and adding HCl in dilute amounts forsome time a very good liberation of Si-grains floating to the surface isachieved. The HCl addition has the effect in addition to the refining ofSi, that the powder mixture does not remain sticky. In this manner it ispossible to obtain a concentration of Si of 80% or more than 80% in aSi/electrolyte grain mixture with a sand-water consistency. Thissand-water consistency has the effect that the mixture is easy to filterand is washed with water and dried at room temperature. As a consequenceof the concentration of Si to 80% in the powder mixture the use of jigas a separator (WO 97) becomes superfluous. What happens is that theacidic mixture gradually reacts with the electrolyte and dissolves it.The Si-grains which are partly embedded in electrolyte, are graduallyliberated and get in contact with the acid/water mixture. The acidicwater attacks the contaminations in Si, which primarily consist ofmetals. Hydrogen gas is formed on the surface and in the pores of theSi-grains, which results in an uplift even in very dilute acid. Inaddition to the fact that Si (d=2.3 g/cm³) floats up to the surface ofthe water, the Si-grains will be hanging there until they are scrapedaway from the surface. The refining of the Si-grains has also beenimproved in addition to the concentration, since the acids over a longertime get in better contact with the liberated Si-grains. (The Si-grainsare so pure that one gets below the detection limit for all the elementsanalyzed with microprobe equipment. This means that there is not anyanalysis method which can determine Si purer than about 99.99% as longit is impossible to concentrate Si to ˜100% from a Si/electrolyte grainmixture).

Si may be melted together with Al prepared in the electrolysis (step I),to form Fe-poor, P-poor, low alloyed AlSi-alloys and/or high alloyedSiAl-alloys, which are desired alloys in may connections.

Both the high alloyed SiAl-alloys and the low-alloyed AlSi-alloys may bedissolved in HCl or H₂SO₄. Al goes into solution and “pure”-Si-powder(˜100% and free from electrolyte) is formed. From dissolved Al pureproducts of AlCl₃ and Al₂(SO₄)₃ are formed.

To further concentrate and refine Si from the Si/electrolyte mixtureafter step II, traditional melting and casting methods for Si arechosen. It has been found that the remaining fluoride containing slagproducts which are now less than 20% of the remaining powder mixture (Siand electrolyte) has a refining effect on the remaining contaminationsin the Si-powder during the melting, by mixing well (stirring together)the Si-powder and residual electrolyte after they have melted, so thatsolidified Si in this case is purer than if fluoride containing slag hadnot been present.

With respect to equipment it is suitable that the walls consisting ofgraphite in the electrolysis furnace advantageously can be replaced bySiC or silicon nitride-bound SiC. The walls of the electrolysis furnacedo not have to consist of Si (WO 95, FIG. 2 number 4). Further, Si doesnot have to cover the anode stem, since a current jump does not takeplace between the cathode and anode even when they grow together.

1. A process for preparing highly purified silicon and optionallyaluminum and silumin (aluminum silicon alloy) in the same cell, whereinI. silicate and/or quartz containing rocks are subjected to electrolysisin a salt melt containing fluoride, whereby silicon and aluminum areformed in the same bath, and aluminum formed, which may be low alloyed,flow to the bottom and is optionally drawn off, and II. deposit formedon the cathode is removed from the cathode and crushed, optionallytogether with the remaining electrolysis bath, concentrated sulfuricacid and then hydrochloric acid and water are added to the crushedmaterial, liberated Si-grains float to the surface and are taken out andtreated further as desired.
 2. The process according to claim 1, whereinthe fluoride-containing electrolysis bath contains cryolite.
 3. Theprocess according to claim 2, wherein soda (Na₂CO₃) and limestone(CaCO₃) are used in the electrolysis bath.
 4. The process according toclaim 2, wherein quartz containing rocks are used as starting materialfor the preparation of Si.
 5. The process according to claim 2, whereina rock containing aluminum rich feldspar (CaAl₂Si₂O₈) is used for thepreparation of both aluminum and silicon.
 6. The process according toclaim 2, wherein further treatment takes place by mixing a basic,neutral or acidic fluoride-containing electrolyte into the moltensilicon; slag and silicon are separated; and the silicon iscrystallized.
 7. The process according to claim 1, wherein soda (Na₂CO₃)and limestone (CaCO₃) are used in the electrolysis bath.
 8. The processaccording to claim 7, wherein quartz containing rocks are used asstarting material for the preparation of Si.
 9. The process according toclaim 7, wherein a rock containing aluminum rich feldspar (CaAl₂Si₂O₈)is used for the preparation of both aluminum and silicon.
 10. Theprocess according to claim 7, wherein further treatment takes place bymixing a basic, neutral or acidic fluoride-containing electrolyte intothe molten silicon; slag and silicon are separated; and the silicon iscrystallized.
 11. The process according to claim 1, wherein quartzcontaining rocks are used as starting material for the preparation ofSi.
 12. The process according to claim 11, wherein further treatmenttakes place by mixing a basic, neutral or acidic fluoride-containingelectrolyte into the molten silicon; slag and silicon are separated; andthe silicon is crystallized.
 13. The process according to claim 1,wherein a rock containing aluminum rich feldspar (CaAl₂Si₂O₈) is usedfor the preparation of both aluminum and silicon.
 14. The processaccording to claim 13, wherein further treatment takes place by mixing abasic, neutral or acidic fluoride-containing electrolyte into the moltensilicon; slag and silicon are separated; and the silicon iscrystallized.
 15. The process according to claim 1, wherein furthertreatment takes place by mixing a basic, neutral or acidicfluoride-containing electrolyte into the molten silicon; slag andsilicon are separated; and the silicon is crystallized.
 16. The processaccording to claim 15, wherein the fluoride-containing electrolyte isthe acidic fluoride-containing electrolyte.